TFTs are widely used for LCD panels. In such a TFTLCD system, each picture element (pixel) has LCD device and a switch to turn the LCD device on and off. A matrix of pixels are placed at the cross-points of a number of rows of sequential scan signals and a number of columns of data signals. When a scan signal and a data signal is coincident at a certain cross-point, the pixel at that particular cross-point is activated. The coincident addressing of this particular pixel is accomplished by a TFT, where the scan signal may be applied to the gate of the TFT and the data signal may be impressed on the drain of the TFT and driving the corresponding LCD from the source of the TFT.
There are a number of structures for TFTs as described in a paper by M. Akiyama et al, "An a-Si TFT with a New Light-Shield Structure and Its Application to Active Matrix Liquid Crystal Displays" IEEE International Electron Devices Meeting Proceedings pp. 268-271 (December 1988). In general, FIG. 1 shows the cross-sectional views of the conventional amorphous silicon (a-Si) TFTs. The table under the cross-sectional views is the comparison among the different kinds of TFTs.
The fabrication processes of type A and type B a-Si TFTs are as follows:
(1) Deposit a metal film as the gate of the TFT on a transparent substrate. PA1 (2) Deposit a-Si, silicon nitride (a-SiN), heavily-doped a-Si (N+ a-Si) films on the substrate. PA1 (3) Etch the N+ a-Si and a-Si films except the active region of the TFT by the standard photolithographic processes and dry etching. PA1 (4) Open the contact holes of the TFTs. PA1 (5) Form the source and drain contact metal of the TFT. PA1 (6) Etch the N+ a-Si layer between the source and the drain electrodes by dry etching.
Because there is no etching stopper in the type A and type B TFTs, step 6 is controlled by the etching time, which is critical, and the thickness of the a-Si layer must be much thicker than that of the N+ a-Si layer. Typically, the thickness of the a-Si layer is more than 2000 Angstroms. Type A and type B TFTs have the same structure except that in the type A TFT, the a-Si layer protrudes beyond both edges of the gate electrode, as described by Sakamoto et al in paper, "A 10-In.-DIAGONAL ACTIVE-MATRIX LCD ADDRESSED BY a-Si TFTs", Proceedings of the SID, Vol.28/2, pp.145-148 (1987).
In the type B TFT the a-Si layer is located completely inside the shadow of of the gate electrodes. When this device is operated in the back gate illumination condition, leakage current is observed in the type A structure, because carriers are generated in the illuminated protruded region due to photoelectric effect. Thus, the type A TFT cannot be used in the TFTLCD. For the type B structure, the a-Si layer is totally shielded by the gate electrode. Thus, there is no photocurrent when it is operated in the back gate illumination condition. However, during the fabrication, the a-SiN layer, i.e. the gate insulating layer, beyond the active region is attacked during the N.sup.31 a-Si etching step (Step 3). Therefore, the yield of the type B structure is very poor when it is used for the TFTLCD which is a matrix array of a large number of pixels.
In order to improve the yield of the TFT, an a-Si TFT which has a second layer of a-SiN has been developed as shown in FIG. 1C. The fabrication process of the type C device is similar to that of type A and type B, except that the top nitride (a-SiN) layer is deposited after the deposition of the a-Si film and the top a-SiN film and the top a-SiN layer is removed from the source and drain contact regions before the deposition of the N+ a-Si layer. The top a-SiN layer remains in the channel region of the transistor, and can be used as the etching stopper during etching of the N+ a-Si layer between the source and drain electrodes because the SiN is resistant to Si etch. The thickness of the a-Si layer can be made very thin, typically less than 500 Angstroms. Due to the low photon absorption in the thin a-Si layer, the a-Si layer can protrude outside both the edges of the gate electrode without incurring substantial amount of leakage current. Since the gate insulating a-SiN layer is not attacked during the formation of the active region, the type C device has a higher manufacturing yield than the type B device.
In the type A and type B devices, the channel length is equal to the space between the source and the drain electrodes. In the type C device, the channel length is equal to the length of the top a-SiN and is longer than the space between the source-drain electrodes. Thus, if the same design rule is used, the channel length of the type C device must be longer than that of type A or type B devices. Thus, the type C device occupies a large area, and is not suitable for high resolution displays. The detailed discussion of this effect is described in a paper by H. Katoh, "TFT-LCD Technology Achieves Color Notebook PC", Nikkei Electronics ASIA, Apr., pp.68-71 (1992).